The Association of American Railroads (A.A.R.) has approved several rail wheel diameters for railroad vehicles used in the United States and contiguous countries. Commonly used in freight service are 28-inch, 33-inch and 36-inch diameter wheels, which constitute the nominal diameters of the cylindrical or cylindrical-conical surfaces that contact the rails. These diameters do not relate to the diameter of the flange that guides the wheels along the rails. Moreover, it is well known that these diameters change as a wheel wears following continuous service.
Brakes for railway cars include brake shoes mounted to brake heads wherein the shoes include a wheel engaging surface that engages the cylindrical or cylindrical-conical surfaces of the wheels. Until recently the metal shoes have been cast metal and known as "metallic" shoes. They are now generally of composition material and known as "composition" shoes. These shoes are primarily non-metallic in that they consist largely of non-metallic friction materials bonded together by various types of resinous matrix materials, although sometimes metallic particles are mixed in with the friction materials. A shoe assembly includes a layer of friction material bonded to a metal backing plate. It therefore may be appreciated that both "metallic" and "composition" shoes are used in railway car braking.
The geometric configuration for composition shoes is prescribed by the Association of American Railroads, and per A.A.R. specifications, even though a variety of wheel diameters exist on the railway vehicles in use, the wheel contacting face or surface of the shoes is limited to one basic radius. Likewise, the contour of the brake head which holds the brake shoe is also prescribed by the Association of American Railroads and includes one basic radius for the brake shoe engaging surface.
Because of the requirements for changing brake shoes at any rail yard facility where the railway vehicles might pause, and the desire for standardization to avoid misapplication of incorrect shoes, as above only one standard non-metallic shoe is available and which has a wheel engaging surface radius of 18 inches. While shoes having an assortment of radii would be preferable, the considerations of simplicity of supply and the hazard of misapplication have caused the standardization favoring the 36-inch wheel.
The development of 28-inch wheels for railway cars hauling high cargo such as road trailers and containers has caused problems where the brake shoe for the 36-inch wheel is used because it may prematurely fail. Application of a 36-inch brake shoe mounted on a standard brake head will at first only effect a narrow band of contact with a 28-inch wheel near the center of the shoe during braking. This causes an air gap or an area of non-contact to exist between the brake head and the shoe which generally coincides with the area that is not contacted at the shoe face by the wheel, resulting in causing the shoe to vibrate during braking until the shoe is worn to a radius conforming to the wheel diameter. This may require considerable time, and it has been known that effective reshaping of the shoe face sometimes requries over 20,000 miles of railway car travel. During this reshaping the vibration can cause premature failure of the shoe that has been particularly evident in the modern composition shoe. Such failure is manifested in cracking of the composition material and sometimes separation of large parts of the material from the backing plate. In some cases cracking and fracturing of the backing plate is experienced. It can readily be appreciated that such premature failure is economically wasteful and dangerous.